The last operation in manufacturing TNT is solidification of the TNT into a relatively safe form that accommodates storage/shipment and subsequent reprocessing. The safest form has been found to be flakes as compared to liquid, granules, or blocks. The flake operation consists of a rotating water cooled drum which intercepts the surface of liquid TNT (110.degree. C.). A 0.030-0.040 inch film of liquid adheres to the drum surface, cools is solidification and is removed by a knife. The resulting flakes are collected and the liquid TNT is replenished so as to provide a continuous process. Safe operation of the process and safe handling of the finished product essentially depends on the solid state of the TNT at the knife. The transition state will cause varying degrees of clumping and secondary crystalline growth increases the density of the finished product and thereby increases the hazard associated with handling TNT. Of particular concern is the unwanted growth of single crystal "whiskers" that can initiate an explosion when broken. To maintain the safety factor associated with the process, it is necessary to monitor the state of the TNT i.e., whether it is in liquid or solid form, prior to flaking and to shut down the process when a threshold of liquidity is measured.